A water-cooled internal combustion engine such as an automotive water-cooled engine is designed so that a water jacket (i.e., coolant circulating path) is formed around a cylinder bore wall. A spacer (water jacket spacer) is normally inserted into the water jacket to control the flow of the coolant.
The water jacket spacer is normally produced by injection-molding a specific resin material. Patent Literature 1 discloses a water jacket spacer that consists of a plurality of segments.
The flow of the coolant that flows through the water jacket may be controlled, and the temperature distribution of the bore wall may be optimized by disposing the water jacket spacer in the water jacket on the side on which the coolant enters the water jacket, without disposing the water jacket spacer on the side on which the coolant is discharged from the water jacket, for example. In such a case, the water jacket spacer is molded to have such a shape that the water jacket spacer can be partially inserted into the water jacket.